In the coating of photographic layers on a support such as a film base or paper, a plurality of individual layers are often coated on the support simultaneously, with each successive layer being superimposed on the layer below by means of a coating hopper. One type of coating hopper, known as a multiple slide hopper, is comprised of individual slide elements which are separated by slots and cavities. By introducing each coating liquid into a cavity, the liquid stream is distributed to the desired width and then metered uniformly across the coating width by flowing through the narrow slot. Upon exiting the slot, the layer flows by gravity down the inclined slide surface. Layers of coating liquids then become superimposed on one another as layers from upstream slots flow over the layers exiting from the downstream slots. At the end of the slide surface, the liquid flows onto and coats the moving web. One method for continuously coating thin layers of a liquid composition on a moving substrate such as a continuous web is the so-called curtain coating method. An early description of a curtain-coating method and apparatus for use in the manufacture of photographic film and paper is found in the patent to Hughes, U.S. Pat. No. 3,508,947, incorporated herein by reference. This and subsequent patents relating to curtain coating (e.g., U.S. Pat. No. 3,632,374 and U.S. Pat. No. 4,287,240) describe the use of a coating hopper to form a free-falling curtain of liquid photographic coating composition which impinges transversely across a moving web of film or paper and forms a coated layer thereon.
Another method of coating liquid layers is bead coating, as disclosed, for example, in the patent to Beguin, U.S. Pat. No. 2,681,294, the patent to Mercier et al., U.S. Pat. No. 2, 761,419, the patent to Russell, U.S. Pat. No. 2,761,791 and others, is a valuable process and incorporated herein by reference.
In coated photographic products insoluble particulates from gelatin sources can cause physical coating defects in the liquid layers. The presence of physical coating defects results in product waste, or if undetected release of lower quality product. To prevent coating of insoluble particulates from gelatin sources, filtration is used at multiple stages in the manufacture of coated photographic products. In photographic products gelatin is a large component of photographic melts on a per weight basis. Therefore the concentration of gelatin insolubles (gel slugs) present in photographic gelatin can dramatically effect the frequency of physical coating defects. To date efforts to reduce the number of gel slugs in photographic gelatin have focused on reduction of gel slug formation during the gelatin manufacturing process and removal of gel slugs from gelatin solutions by filtration. Reduction of gel slug formation during gelatin manufacturing can be achieved to some degree through optimization of drying conditions, however to date no gelatin manufacturer has been able to completely eliminate gel slugs. Although filtration of gelatin solutions can reduce gel slug levels this method increases process cycle time and is costly due to filter changes.
An object of the present invention is to reduce gel slugs or gelatin insolubles in a gelatin solution.
Another object of the present invention is to provide a gelatin having reduced gelatin insolubles.